Ok, Im not sure if there is a bad ground or some kind of shared power here, but this is the issue.
the stereo, aftermarket, crackles (but only on radio,not cd) when i turn on the air or fan. anyone ever have this problem?

 

There is a TSB for this, take it to the Dealer.

 

not a dealer issue with a after market radio. You may need an interference filter.

 

Sorry read right thru aftermarket

 

The antenna was used as a ground by GM. Maybe something there?

 

It does sound like GM Used the antenna as a ground for some of the cars systems
and with the aftermarket stereo, i can kiss the warrantee on that good by.